This invention relates to a routing system for use in a data communication exchange network in selecting an alternative route from a plurality of transit trunks of the network with no interruption and, more particularly, to a routing system for use in a data communication exchange network operable with X.25 protocols.
The data communication exchange network is a data communication network comprising a plurality of exchange or switching stations. The transit trunks connect each station to others of the exchange stations.
In the manner which will later be described in greater detail, each transit trunk provides a group of trunk channels. Each station comprises a group of channel-per-station unit connected to the trunk channel group as a trunk terminator group. The station consequently comprises trunk terminator groups which physically terminate the transit trunks to receive incoming signals from the other exchange stations and to send outgoing signals to the other exchange stations through the tranist trunks. The trunk channels of the transit trunk groups are in one-to-one correspondence to the incoming signals and to the outgoing signals. Each of the incoming and the outgoing signals includes a data call.
When a fault takes place either in one of the trunk channels of the transit trunks or in the exchange station connected to one of the transit trunks, the trunk channel is a faulty channel transmitting the fault by its incapability of transmitting a corresponding one of the incoming signals and a corresponding one of the outgoing signals. Merely for convenience of the description which follows, attention will be directed to the transit trunks, trunk by trunk, rather than to the trunk channels. Including the faulty channel, the transit trunk will therefore be called a faulty trunk.
It is known to detect the fault by monitoring the incoming signals received through the transit trunks, to locate the faulty trunk, and to select an alternative route through the transit trunks which are not faulty but normal trunks. The alternative route is used in place of the faulty trunk in order to continue reception of the incoming signal and transmission of the outgoing signal with no interruption. In the meantime, the fault is restored or recovered to reuse the faulty trunk as a normal trunk.
Various manners of detection of the fault or the faulty trunk and of restoration of the fault are known.
A first of these manners resorts to fault detection/restoration in an open systems interconnection (OSI) layer 1. In accordance with this first manner, the fault is detected add restored by monitoring a carrier signal or a like control signal received from a modem connected to each transit trunk.
A second manner is applied to a data communication network with X.25 protocols used between the network and communication terminals connected to the exchange stations and resorts to fault detection/restoration is an open systems interconnection layer 2. The fault is detected and restored by monitoring a disorder in sequence numbers defined by the layer 2 for retry of transmission, time out of timer control, or an error detected by a flag check sequence (ES).
A third manner is to use a frame relay. That is, the fault is detected and restored by receiving frames which the network automatically generates for delivery to the exchange stations in the incoming signals in order to indicate a state of congestion and/or fault according to the consolidated link layer management (CLLLM) messages defined by the frame relay. The fault is detected and restored furthermore by a local management interface (LMI) to monitor whether or not a link is normal between the network and each user.
The frame relay is a protocol which makes it possible to transmit data at a higher rate than packed-switch exchange. This is rendered possible by processing a core function alone in the layer 2 to detect transmission errors and the like by the flag check sequence between the network and the user terminals and thereby to reduce protocol processing necessary on the side of the network. The timer control and others of the layer 2 and of layers 3 or higher are left to or given in charge of processing between the users.
A fourth manner is to use in fault detection/restoration a status signal indicative of whether or not the transit trunks are normal. This manner is disclosed in Japanese Patent Prepublication (A) No. 13,139 of 1990 in connection with a satellite communication network and a terrestrial communication network.
Various manners are known also as regerds control of the transit trunks during presence of the fault.
A first of these manners will herein be called an A manner. According to the A manner, use is made of a redundancy, namely, a communication network comprising two series, such as No. 0 and No. 1 series, as an active and a standby series. When a fault takes place in the active series, the standby series is used to continue communication with no interruption.
A second of the manners will be called a B manner, wherein a routing table is used to specify it least one additional transit trunk in the communication network comprising either only a single series or two series. When the fault occurs to disable communication, the routing table is referred to for selection of the alternative route through the additional transit trunk or trunks.
A third of the manners will be called a C manner, wherein the routing table is also used. When the fault occurs in whichever of the single series, one of the two series, and one of the transit trunks to give rise to failure of a call, the routing table is used in afresh directing the call to an additional transit trunk as a fresh call and in using the additional transit trunk as the alternative route.
When this C manner is applied to a data communication network wherein X.75 protocols are used as protocols of the transit trunks, each exchange station must comprise an LAPB LSI in supporting the X.75 protocols. Such LSI's are separately used for the normal transit trunks and for the additional transit trunk or trunks.
In the manner described in the foregoing, conventional data communication networks have been complicated, redundant, and/or expensive. This is because the fault detection/restoration is carried out for open systems interconnection layers above the layer 3 per transit trunk and necessitated separate hardware and individual software for the hardware for each transit trunk.
In addition, attention should be directed to a problem as follows. When the X.25 protocols are used in the C manner for a duplicated communication network, a call from an X.5 communication terminal may be interrupted by an error in the sequence numbers and time out of the timer control to disable trial of retransmission of the call. Such an interruption takes place when the active series is switched to the standby series, when the faulty trunk is switched to a normal trunk, or when a next call is initiated to the alternative route.
In such an event, the LAPB LSI is used as above for the X.75 protocols. This necessitates LAPB LSI's individually for the normal trunks and for the alternative route or routes and renders the data communication network expensive.